Metal Finishing Guide Book

2012-2013

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strate mix being treated. Minimization of hydrogen development occurs by the use of oxidation agents (Ox) (accelerators). Steel surface Fe + 2H+1 + 2Ox ��� Fe+2 + 2HOx Zinc-coated steel surface Aluminum surface Zn + 2H+1 + 2Ox ��� 2HOx Al + 3H+1 + 3Ox ��� Al+3 + 3HOx Coating Reaction The second reaction that occurs is the coating reaction. Because of the consumption of the free acid in the liquid-metal interface, pH rises and the metal cations cannot stay soluble in the solution. They react with the phosphate in the solution and deposit on the metal surface as crystalline zinc phosphate. Depending on the process chemistry involved, several crystal structures are possible: 3Zn+2 + 2H2PO4-1 + 4H2O ��� Zn3(PO4)2.4H2O + 4H+ 2Zn+2 + Fe+2 + 1H2PO4-1 + 4H2O ��� Zn2Fe(PO4)2.4H2O + 4H+ 2Mn+2 + Zn+2 + 2H2PO4-1 + 4H2O ��� Mn2Zn(PO4)2. 4H2O + 4H+ 2Zn+2 + Mn+2 + 2H2PO4-1 + 4H2O ��� Zn2Mn(PO4)2. 4H2O + 4H+ Sludge Reaction The metal ion (Fe+2) that is dissolved from the pickling reaction is oxidized using the accelerator (e.g., nitrite/nitrate, chlorate, or peroxide) and will precipitate out as sludge. In the case of zinc ions (Zn_2) they are incorporated in the coating reaction and will not form any sludge. Aluminum ions (Al+3), by the use of fluoride, react to form a fluoroaluminate complex. This complex is precipitated as kryolith. Fe+2 + H+1 + Ox ��� HOx Fe+3 Fe+3 + H2PO4-1 ��� FePO4 (sludge) + 2H+ Al+3 + 6F-1 ��� AlF6 -3 + AlF6 3Na+ ��� Na3AlF6 (sludge) The created sludge in the process is normally filtered out from the solution utilizing some sort of filter media or equipment, such as lamella clarifiers or settling tanks, direct feed through a filter press, or hydromation filter technology. The system to be used is a matter of discussion and preference of customer or line manufacturer. ZINC PHOSPHATING PRIOR TO PAINTING To increase the durability and service life of painted parts where maximum performance is essential, zinc phosphating is widely used. Today���s zinc phosphate 121

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